Transmission of electrical signals having a direct current component



Jan. 5, .1943. A. DQBLUMLEIN 2, 0 TRAfiSlIISSION OF ELECTRICAL SIGNALS HAVING'A DIRECT CURRENT COMPONENT Filed lia ze, 1939 INVENTCR ALAN DOWE B MLE/N ATTORNEY I Patented Jan. 5, 1943 TRANSMISSION OF ELECTRICAL SIGNALS HAVING A DIRECT CURRENT COMPO- NENT Alan Dower Blumiein, Ealing, London, England, assignor to Electric & Musical Industries Limited, Hayes, Middiesex, England, a company of Great Britain Application May 26, 1939, Serial No. 275,823 In Great Britain May 30, 1938 Claims. (Cl. 178-73) The present invention relates to apparatus for handling electrical signals having a direct current component, and is concerned with improvements or modifications of the invention described in the parent specificationU. S. application Serial No. 69,831, now Patent No. 2,224,134, issued December 10, 1940.

In the parent specification there is described a method of correcting for variations in the efiective amplitude of electrical signals representative of intelligence, such as may arise in the transmission of said signals as a result of the complete or partial loss of the D. C. component of said signals, the incorrect representation of that component, or varying attenuation of the signals, the said method comprising transmitting at spaced time intervals along the channel through which the intelligence signals are passed, check signals each of which has a switching portion and a datum portion, said datum portion having, at the input of said channel, either a predetermined fixed amplitude value or a predetermined wave form comprising fixed amplitude values, and the said method being characterised in that the datum portions are applied through one path to an observingdevice, while the switching portions, or switching signals derived therefrom,

are fed to said observing device through another path, and serve to change said observing device from the inoperative condition into the operative condition, said observing device, when in the operative condition, serving to develop a corrective signal dependent upon the amplitude or amplitude and waveform of the datum portions applied thereto, and said corrective signal being applied at a point either before or after the observing point to compensate wholly or in part for said variations in eflective amplitude.

There is described in said parent specification with reference to Fig. 7 of that specification and which is embodied in this application as Fig. 3, a method of developing corrective signals wherein the corrective signals are applied to a point prior to the observing point in order to correct for any errors in the direct current component, but not to re-insert the direct current component, the latter being carried out by other means.

Briefly, the D. C. component is re-inserted in a television signal, by means of the method described in the specification of U. S. application Serial No. 720,205, now Patent No. 2,252,746, issued August 19, 1941, at the grid of a modulator valve. The resultant modulated carrier wave is rectified and the resultant D. C. datum portion is observed and the corrective signal is passed from the observing device back to the re-insertion device, thus correcting for any variation in the datum level in the transmitter output.

It is the object of the present invention to provide improvements in or modifications of the invention described in parent specification'U. S. application Serial No. 69,831.

According to the invention a method ofcorrecting for variations in the eflective amplitude of electrical signals representative of intelligence, such as may arise in the transmission of said signals as a result of the complete or partial loss of the D. C. component of said signal according to a modification of the showing of the parent patent U. S. application SerialNo. 69,831, wherein said signals are applied to a. direct current amplifier and said switching portions, or switching signals derived therefrom, are fed to said observing device from a point prior to or subsequent to said direct current amplifier to switch the observing device from the inoperative to the operative condition to observe the datum portions after the latter have passed through said direct current amplifier and developing a correcting signal from the datum levels observed by said device, which correcting signal is fed back to said direct current amplifier in such a manner as to cause said datum portions to have a substantially constant value at the output oi. said direct current amplifier.

According to one arrangement of the invention, television signals are applied to the input of a direct current amplifier and the amplified output signals are applied to one grid of a hexode valve forming the observing device and delayed synchronising pulses are applied to the other grid of the hexode valve so that the hexode valve is only switched on during the datum portions of the television signal which correspond to black. The resultant pulses of anode current in the hexode valve produce potential pulses which are rectified and the rectified signal is fed back to the input of the direct current amplifier.

In order that the said invention may be clearly understood and readily carried into eifect, it will now be more fully described with reference to the accompanying drawing which illustrates particular embodiments according to the invention, and in which Fig. 1 shows one embodiment thereof;

Fig. 2 is a modification thereof; i

Fig. 3 is the showing of Fig. '7 of the parent application;

Fig.4 is the showing of Fig. 8 of the parent application.

Referring to Fig. 1 which shows one embodiment of the invention, television signals which may be of the form shown in Fig. 8 of the parent specification and Fig. 3 of this application are applied at terminal 20! with or without the D. C. component present and are passed through the A. 0. coupling consisting of condenser 202 and resistance 203. The signals are then amplified by the direct current amplifier 204 and pass out at line 205. It is assumed in the arrangement shown that the amplifier 204 gives an output signal in the same sense as the input signal. In addition, it is assumed that the television signals are applied at the terminal 20! with the picture signals in the positive sense and therefore the signals in the output at 205 have also picture amplitudes in the positive sense. The amplified signals at line 205, which may be of quite large amplitude, are applied through a potentiometer to the grid of a phase reversing valve 206, in the anode circuit of which is a delay network 201 from which the signals are applied through a condenser and leak resistance to the inner grid of the hexode valve 208 which forms the observscreening grid is supplied with a suitable positive potential. The signals are applied through a potentiometer to valve 205 in order to prevent this valve from overloading. The delay network 201 comprises the usual series inductances and shunt capacitances and the anode resistance of valve 200 serves to terminate the delay network.

The delayed synchronising signals applied to the inner grid of the hexode are in a positive sense, and are of such an amplitude as to exceed the grid base for the inner grid of the hexode 208. As explained in the specification of U. S. application Serial No. 720,205, the D. C, component of the signal will be re-established at this point and the valve will only pass current during the periods when these (delayed) synchronising pulses are applied to the inner grid of the valve. The synchronising signals are delayed so that when they arrive at valve 208 they coincide with the arrival of the datum portion 13 representing black (see Fig. 8) on the outer grid of hexode 208 which grid is connected to line 205. If the duration of the synchronising pulse P (see Fig. 4) is shorter than the duration of the datum portion B, then P can be delayed to lie totally within B.' If P is longer than B, the delay should be slightly less than the duration of B. The fact that the hexode 208 will be switched on before the occurrence of the datum portion B does not matter, since during this time the signals on the outer grid will correspond to P, that is, they will be very negative and so will prevent the valve passing any anode current. During the occurrence of the datum portion B the inner grid of the hexode valve will be switched on. The absolute potential of the outer grid during this period will be the absolute value of B, and so the anode current of the valve will be determined by the difference of the absolute value of the grid potential and the potential of the cathode battery 209, which in effect provides a reference potential. Of course, this reference potential may be zero if the desired level of the datum portion B at lead 205 is close to zero. Since during periods other than the datum period B the valve 208 will be switched off, the anode current will consist of a series of pulses, the absolute value of which depends on the difference between the absolute value of the potential of the datum portion B at 205 and the potential of the battery 200. The signal ampli- 5 the valve 208 produce corresponding potential pulses, which are applied to the rectifiers 2|. (shown as diode valves operating in a voltage doubler type of circuit) with a condenser and leak resistance circuit 2, which generates a negative bias at the input of the amplifier 204,

tending to make the output signal negative. Any positive drift of potential of the datum portion B at the lead 205 will increase the output of the hexode 208 and hence the negative input bias of amplifier 204 is increased, thus counteracting the drift of the output.

As an example of the operation of the circuit, suppose that the mutual conductance of the outer grid of hexode 208 to the anode is 1.0 milliampere per volt and the anode resistance of valve 200 has a value of 2000 ohms; suppose also, that the rectifier valves 2 I 0 are 80% efllcient and that the amplifier 204 has a gain of 100. Now, if the D. C. level, that is, the absolute level of the datum ing device. The valve 200 is a pentode and the portion B at the input of the amplifier 204 tends to drift, due to the coupling 202 and 202, by 0.8 volt, which requires 0.8 volt from the rectifier 2|0 to compensate for the drift, then a change of pulse amplitude of 1 volt from the valve 200,

that is, 0.5 milliampere change of current or.0.5

volt change of potential on the outer grid of the hexode is required. The datum level of B would therefore only change at line 205 by about 0.5 volt in place of 0.8x 100, that is, 80 volts had there been no corrective device. The battery 2I2 is inserted between. the rectifiers 2l0 and the leak resistance 203 to ensure that the bias at the input of amplifier 204, in the absence of any output from rectifiers 2 I0, is always too positive so that it can be corrected by the rectifiers.

The above arrangement has been described with reference to an amplifier which produces an output signal in the same sense as the signal applied to the input. The circuit arrangement of Fig. 1 may be modified to suit other conditions, for example, if the picture signals are applied to the input of amplifier 204 in a negative sense and the amplifier is such that a positive picture output is obtained, then the rectifier diodes 2 l0 must be reversed so that they generate portion B, it will be necessary to generate short pulses by such a circuit as shown in Fig. 12 of the parent specification. Similarly, as described with reference to Fig. '7 of the parent specification and which is Fig. 3 of this application, the

amplifier 204 may include a carrier frequency modulator and in this case a rectifier is required between line 205 and the connections to valve 205 and valve 208. Referring to Fig. 3, if the diode valve 55 is omitted and the leak resistance 54 re 5 places it between the grid of valve 52 and the lead 56, the circuit of Fig. 1, together with the other apparatus described in connection with it, provides one example of this method of D. C. re-insertion applied to the output circuit of a radio transmitter.

A further modification of the invention is shown in Fig. 2. In this figure the direct current amplifier 204 has two input circuits; for example, the input circuit of the amplifier may comprise two valves, the anodes of which are connected in parallel and provided with a common output impedance, and the input grids of these two valves provide the input terminals for the amplifier. It is also assumed in connection with this figure that the amplifier 204 produces a phase reversal between the input and output circuits. In addition, it is assumed that the amplifier 204 produces a potential of substantially zero volts at the output 205 when the bias applied to both in-- puts of the amplifier is zero. Due to the presence of the positive bias battery N2, the output at line 205 tends to be very negative unless corrected by the negative bias produced by rectifier 2l0, which, in this case, is represented in a. halfwave rectifier circuit. The two valves 2 and U form the observing device and are provided with a large common cathode resistance which is taken to a source of negative potential. The anodes of both these valves are taken to suitable positive potentials, the grid of valve 2l5 being connected directly to the line 205 and the grid of valve 214 being connected via. a coupling condenser and grid resistance to the device 213 which represents an arrangement for producing negative pulses on the grid of valve 2l4 during only those periods when line 205 is at datum potential, that is, during the datum periods. Such pulse producing devices are described in the parent specification and are illustrated, for example, in Fig. 12 of that specification. Since such devices are known heretofore they are not illustrated in this specification. The cathode of valve 2 is normally biased positively with respect to the potential of line 205 so that the cathode of valve H5 is held positive irrespective of the potential on its grid, thus keeping diode 2| 0 from conducting. The negative pulses on the grid of valve 2 l4 cause the cathode potential of this valve to become negative until valve 2l5 conducts. The cathode potential and thus the output potential of valve 2 I 0 then depends on the potential of the grid of valve 2l5 during the datum period. The potential stored in condenser 216 is passed to the input of the amplifier 204 so as to correct the output datum level to a. value substantially equal to zero volts (or slightly negative of zero due to the bias required by valve The time constant of condenser 216 and its leak resistance must be long compared with the intervals between successive datum portions but must be short enough having regard to the fact that the potential across it is, in efiect, amplified by amplifier 204, to enable correction to be made for the errors in the D. C. component of the input signal. Suitable values for the'components for the diode load impedance can be found experlmentally, although, of course, correction by means already described can only be made for changes in the D. C. level of the input signals which are slow compared with the interval between successive datum periods. This also applies to the condenser and leak resistance of the rectifiers 2I0 in Fig. 1.

The arrangement described above can be used omitting condenser 202 and resistance 203 for an input signal which contains the D. C. component to correct for any errors in the input or to correct for any errors in the D. C. amplification of amplifier 204.

The arrangements of Figs. 1 and 2 can also be operated with the switching pulse derived from the input, or from an intermediate point in the amplifier. For example, in Fig. 2 the device M3 may receive its input from the input to amplifier 204. In this case an amplifier may be required in the device H3 in order to generate sufliciently large pulses. This arrangement has the advantage that any faults in the amplifier 204 which might overload it and reduce the amplitude of the switching pulses cannot affect the D. C. re-insertion. Such reduction in the amplitude of the synchronising pulses might be a disadvantage with the arrangement of Fig. 1 when first switching on. This can always be corrected, however, by a manually controlled bias applied to a resistance 203.

We claim:

1. Apparatus for re-inserting the direct current component in a signal in which said component has been diminished or lost and wherein said signal includes a datum portion and a recurring portion, means for amplifying said signal, a multi-grid vacuum tube having anode, cathode and at least two grid electrodes. said rnulti-grid tube comprising an observing device, means for impressing the datum portion of said signal onto a control electrode of saidtube, means for substantially simultaneously impressing said recurring portion ofsa-id signal onto a control electrode of said tube to render said tube conductive and capable of measuring the absolute value of said recurring datum portions, means for storing energy in accordance with the output of said tube, and means for impressing a corrective signal onto said amplifier in accordance with the value of said stored energy.

2. Apparatus in accordance with claim 1, wherein there is provided in addition delay means for delaying the impression of the recurring portion of said signal onto said multi-grid tube.

3. Apparatus in accordance with claim 1, wherein said means for storing energy inaccordance with the output of said multi-grid tube comprises a pair of rectifying means connected substantially in parallel with a condenser.

4. Apparatus in accordance with claim 1, wherein said means for storing energy in accordance with the output of said multi-grid tube comprises a pair of rectifying means connected substantially in parallel with a condenser, and wherein said rectifying means comprise diodes. 5. Apparatus for -re-inserting the direct current component in a signal in which said component has been diminished or lost and wherein said signal includes a datum portion and a recurring portion, means for amplifying said signal, a multi-grid vacuum tube having anode, cathode and at least two grid electrodes, said multi-grid tube comprising an observing device. means for impressing the datum portion of said signal onto a control electrode of said tube. means for changing the phase of the recurring portion of said signal, time delay means,means for impressing the output of said phase changing means onto said time delay means, means for impressing the delayed recurring portion of said signal onto a control electrode of said multi-zridtube, condenser means, rectifying means connected substantially in parallel with said condenser means, means connecting the output of said multi-grid tube to said rectifying means, and means coupling said condenser means to the input of said amplifier.

ALAN DOWER BLUMLEIN. 

